FIG. 15 is a schematic view of a polishing apparatus for polishing a wafer. As shown in FIG. 15, the polishing apparatus includes a polishing table 22 for supporting a polishing pad 23 thereon, and a top ring 30 for pressing a wafer W against the polishing pad 23. The polishing table 22 is coupled via a table shaft 22a to a table motor 29 disposed below the polishing table 22, so that the polishing table 22 is rotated by the table motor 29 in a direction indicated by arrow. The polishing pad 23 is attached to an upper surface of the polishing table 22, and an upper surface of the polishing pad 23 provides a polishing surface 23a for polishing the wafer W. The top ring 30 is secured to a lower end of a top ring shaft 27. The top ring 30 is configured to be able to hold the wafer W on its lower surface by vacuum suction.
Polishing of the wafer W is performed as follows. The top ring 30 and the polishing table 22 are rotated in directions indicated by arrows, respectively, while a polishing liquid (or slurry) is supplied from a polishing liquid supply structure 25 onto the polishing pad 23. In this state, the top ring 30, holding the wafer W on its lower surface, is lowered and presses the wafer W against the polishing surface 23a of the polishing pad 23. A surface of the wafer W is polished by a mechanical action of abrasive grains contained in the polishing liquid and a chemical action of the polishing liquid. The polishing apparatus having such structures is known as CMP (Chemical Mechanical Polishing) apparatus.
A pressure chamber (not shown in FIG. 15) that is defined by an elastic membrane is provided at a lower portion of the top ring 30. This pressure chamber is supplied with a pressurized gas so that a polishing pressure applied to the wafer W on the polishing pad 23a is adjusted. FIG. 16 is a schematic diagram showing a pressure regulator 100 for regulating the pressure in the pressure chamber of the top ring 30 by supplying a gas (air, nitrogen, or the like) into the pressure chamber. As shown in FIG. 16, the pressure regulator 100 includes a pressure regulation valve 101 for regulating the pressure of the gas supplied from a gas supply source, a pressure sensor 102 for measuring the pressure (secondary pressure) of the gas downstream of the pressure regulation valve 101, and a regulator controller 103 for controlling operation of the pressure regulation valve 101 based on a pressure value obtained by the pressure sensor 102. The pressure regulator 100 having such structures is known as an electropneumatic regulator.
The pressure regulation valve 101 has a pilot valve 110 for regulating the pressure of the gas supplied from the gas supply source, and a gas-supply solenoid valve 111 and a gas-exhaust solenoid valve 112 each for regulating the pressure of pilot air to be delivered to the pilot valve 110. The pilot valve 110 has a pilot chamber 115 partly defined by a diaphragm, and further has a valve element 116 coupled to the pilot chamber 115. The pilot air is delivered through the gas-supply solenoid valve 111 into the pilot chamber 115, and the pilot air in the pilot chamber 115 is discharged through the gas-exhaust solenoid valve 112. Therefore, the pressure in the pilot chamber 115 is regulated by the operations of the gas-supply solenoid valve 111 and the gas-exhaust solenoid valve 112. The regulator controller 103 controls the solenoid valves 111, 112 to open and close, and the valve element 116 moves in accordance with the pressure in the pilot chamber 115. Depending on a position of the valve element 116, the gas from the gas supply source passes through the pilot valve 110, or the gas that is present downstream (secondary side) of the pilot valve 110 is discharged through the pilot valve 110. The pressure of the gas that is present downstream of the pilot valve 110, i.e., the secondary pressure, is thus regulated.
The regulator controller 103 is coupled to a polishing controller 50 of the polishing apparatus, and receives a pressure command value sent from the polishing controller 50. The regulator controller 103 controls the operations of the gas-supply solenoid valve 111 and the gas-exhaust solenoid valve 112 in order to eliminate a difference between a present pressure value of the gas measured by the pressure sensor 102 and the pressure command value to thereby regulate the pressure in the pressure chamber of the top ring 30.
However, the pressure regulator of the above structures is problematic in that either stability of the pressure or responsiveness to an input signal is low. Specifically, if the stability of the pressure is improved, then a response time is lowered, and if the responsiveness is improved, then the pressure becomes unstable.